Titanium dioxide is considered the principal white pigment of commerce. It has exceptionally high refractive index, negligible color, and is quite inert. Titanium dioxide may be present in either of two predominant forms, anatase or rutile. For the majority of commercial applications, rutile is the desired form.
There are two main processes for making raw pigmentary titanium dioxide, the sulfate process and the chloride process. The sulfate process is based on the digestion of ilmenite or titania slag in concentrated sulfuric acid. After iron removal as iron sulfate, the solution is heated and diluted with water. The titanium hydrolyzes, forming a titanium oxysulfate precipitate, which is further treated to produce TiO2 pigment. The chloride process relies on chlorination of low-iron, titanium containing ore or intermediate products to form TiCl4, followed by the gas-phase oxidation of TiCl4.
Alternative methods based on hydrochloric acid leaching of titaniferous ore have been proposed. One example is disclosed in U.S. Pat. No. 3,903,239. There, the process includes the hydrolysis of titanium oxide from a chloride solution by heating and dilution. The hydrolysis step is similar to the hydrolysis procedure conventionally used in the sulfate process.
Hydrolysis by heating and dilution with water presents a number of disadvantages. It creates large volumes of dilute acid solutions that are expensive to recycle and the disposal of which presents environment problems. The very fine, hydrolyzed TiO2 has to be removed from a large volume of solution, generally by filtration. The conditions of hydrolysis have to be adjusted to make the resulting suspension filterable, which limits flexibility in choosing hydrolysis conditions. Chemical control additives can be introduced in solution prior to hydrolysis to influence the characteristics of the product, but the additives are generally not homogeneously distributed over the mass of the TiO2 product, and their effectiveness is less than optimal.
The present invention makes it possible to produce a high quality titanium dioxide pigment from an aqueous titanium solution without the disadvantages mentioned. It is particularly well suited when a titanium solution of high purity is available. For example, the solution can be derived from the process disclosed in U.S. application Ser. No. 60/141,114 filed on Jun. 24, 1999 and from U.S. application Ser. No. 09/500,207, U.S. Pat. No. 6,375,923 the entire contents of each are incorporated herein by reference.
The present invention relates to an economical hydrometallurgical process for producing pigment grade TiO2 from aqueous titanium solutions. The solutions may be derived from any of several sources, but are generally derived from processing mineral ores and, in particular, ilmenite ore. The processing to produce the solutions can be, for instance, a leaching or dissolution process, followed by any of several means for solution purification. For example, the solution can be derived from the process disclosed in U.S. application Ser. No. 60/141,114 filed on Jun. 24, 1999 and from U.S. application Ser. No. 09/500,207, U.S. Pat. No. 6,375,923 the entire contents of each are incorporated herein by reference. Alternatively, the processing to produce the solution can be from a chlorination process where the TiCl4 is dissolved in water or a hydrochloric acid solution. Furthermore, the process according to this invention is not limited to solutions containing titanium and chloride, but titaniferous sulfate and titaniferous nitrate solutions can also be used.
In one embodiment, the solutions are aqueous titanium chloride solutions and are comprised of water, hydrochloric acid, titanium oxychlorides, and titanium chlorides. These solutions may vary widely in composition with the respect to the hydrochloric acid content and the titanium content.
The titanium chloride solutions are first converted to a titanium oxide solid in a controlled temperature, total evaporation process at a temperature higher than the boiling point of the solution, but below the temperature where there is significant crystal growth.
The process includes evaporation of the solution in a controlled manner, hydrolysis to form amorphous titanium oxide, further evaporation and drying of the product. It generally involves the formation of a thin film of titanium oxide. The process is preferably effected by spraying and is referred to herein as spray hydrolysis. Spray hydrolysis can be accomplished in a spray dryer, or any other suitable piece of equipment. The gas phase formed as a result of evaporation, contains acid and water, and can be further processed to regenerate the acid (hydrochloric, sulfuric, or nitric acid).
The titanium oxide is then calcined at an elevated temperature to transform the oxide to a desired crystalline form of titanium dioxide suitable for use as a white pigment. The desired cyrstalline form is usually the rutile form of titanium dioxide but, depending upon the type of chemical additives used, the anatase form can also be produced in this process.
Following calcination, the titanium dioxide is milled to produce the desired particle size distribution suitable for use as a white pigment. The milling may be by wet milling.
Finally, the milled titanium dioxide is dewatered, usually by filtration, and optionally dried, usually by spray drying, to yield a final titanium dioxide white pigment.
The advantages of the process according to the present invention include a high quality titanium dioxide pigment, readily controlled physical and chemical characteristics of the pigment product, and low cost processing, since dilution of the solution is avoided and the acid in the gases can be recycled.